An Ocean Thermal Energy Conversion (OTEC) system generates electrical energy based on a temperature difference between cold seawater at a deep-water region and warm seawater near the ocean surface. Typically, OTEC systems rely upon large, robust heat exchangers that transfer heat between a working fluid and the seawater as part of a Rankine-cycle engine.
In the Rankine cycle, heat from the warm seawater is transferred into the working fluid at heat exchangers configured as evaporators, where the working fluid is vaporized. The warm seawater is pumped from the surface region to the evaporators via a seawater conduit. The vaporized working fluid drives a turbogenerator that generates the electrical energy. After it has passed through the turbogenerator, heat is transferred from the vaporized working fluid into the cold seawater at heat exchangers configured as condensers, where the vapor condenses back into liquid form. The cold seawater is pumped from the deep-water region to the condensers via a cold-water conduit, which is often 1000 meters long or more. In a closed-cycle system, the liquefied working fluid is pumped back to the evaporators to continue the Rankine cycle.
The heat exchangers are sometimes located on a ship or on the deck of an offshore platform, such platforms used in oil drilling, etc. It is preferable, however, that the heat exchangers are submerged below the water line to reduce platform costs and reserve deck space, among other things. In some cases, heat exchangers are housed in submerged compartments that are part of the offshore platform itself.
OTEC heat exchangers must withstand prolonged exposure to the working fluid (e.g., ammonia), as well as a large secondary flow of the seawater—particularly for submerged OTEC heat exchangers. Further, it is highly desirable, if not necessary, that such heat exchangers provide high overall heat transfer coefficients, exhibit minimal mechanical pumping losses, and are light weight. Still further, it is important that the materials and fabrication costs of these heat exchangers are not excessive.
Unfortunately, OTEC heat exchangers are highly susceptible to biofouling, corrosion, and degradation over the operating lifetime of an OTEC system. It is necessary, therefore, to have access to the heat exchangers for regular maintenance, as well as emergency repair and/or replacement. In addition, it is often desirable to upgrade the capability of an OTEC system by, for example, adding heat exchangers, upgrading heat exchangers to increase heat-transfer capacity, etc.
Removal and/or attachment of a submerged heat exchanger can be extremely challenging, however—especially in cases when such operations require personnel to gain access to underwater compartments and/or require special diver operations. In addition, it is often necessary to shut down the OTEC system during maintenance, repair, and upgrade operations. These shutdown periods can have significant impact on the overall production capability of such systems.